Melanoma is a common solid tumor notorious for its high rate of metastasis and resistance to chemotherapy and radiation. Several factors may account for the resistance of melanoma to current therapies. First, melanomas are derived from melanocytes, specialized neural crest cells that are specialized to produce melanin. The production of melanin results in the generation of toxic reactive oxygen species and cytotoxic phenol derivatives, thus melanocytes are equipped with mechanisms to resist these insults. Recently, the microopthalmia gene (MITF), which is a master transcriptional switch of melanocytes, has been shown to possess antiapoptotic activity and is found in metastatic lesions at a high frequency (25) (26). Second, multiple signaling pathways are activated in melanoma. B-raf is mutated in many melanomas, resulting in constitutive activation of MAP kinase signaling (1, 27). N-ras is also mutated frequently in melanoma, resulting in activation of MAP kinase, phosphoinositol-3 kinase/akt signaling, and S6 kinase activation (28, 29). While B-raf and constitutive MAP kinase activation is sufficient to cause transformation of melanocytes into melanoma (3, 4), other signal transduction events are frequently observed in B-raf mutant melanomas, such as loss of the tumor suppressor PTEN (29, 30). The consequences of PTEN loss is activation of phosphoinositol-3 kinase/akt activation.
Multiple regimens have been tried for the treatment of locally advanced and metastatic melanoma. Initial trials several decades ago used agents such as hydroxyurea, and more recent agents used against melanoma include dacarbazine and platinum based therapies including cisplatin and carboplatin. Other therapies, including biochemotherapy, have included interleukin-2 infusion and infusion of lymphocytes which are present in melanoma lesions and have been expanded ex vivo (31). All of these therapies have had modest success in a minority of patients, but with significant toxicity, including pulmonary leak syndrome (32-36). Currently, interferon alpha is employed in high risk patients, and prolonged therapy results in a 10% long term survival benefit.
Targeted therapies have been attempted in melanoma. Sorafenib was developed as a B-raf inhibitor based upon the observation that B-raf mutation is common in melanoma. However, results from initial trials of sorafenib in melanoma have been disappointing (15). Everolimus has also been tried against human melanoma, and has not been successful as a single agent (37). Current knowledge of signaling may provide an explanation of why previous therapies have failed. Phosphoinositol-3 kinase activation has been shown to mediate against extrinsic pathways of apoptosis, which include apoptosis due to TRAIL, TNF alpha, and interferons (38). Monotherapies of these cytokines may be frustrated in the face of phosphoinositol-3 kinase activation. Similarly, apoptosis induced by tumor infiltrating lymphocytes may be frustrated by phosphoinositol-3 kinase activation. Phosphoinositol-3 kinase also activates VEGF expression, and in addition to stimulating angiogenesis, VEGF inhibits dendritic cell function, impairing immune responses to melanoma (39-43).
Targeting MAP kinase as monotherapy in melanoma is clearly insufficient to eliminate melanoma in most patients. MAP kinase is activated in a majority of human melanomas, including those that lack B-raf mutation (3). In a previous study of human melanomas, we demonstrated that a subset of advanced melanomas had decreased MAP kinase activation, implying that additional signaling pathways are operative in vivo. Further support of this hypothesis is our previous finding that treatment of EBV-induced Burkitt's lymphomas with antioxidants resulted in compensatory MAP kinase activation (17). It is likely that treatment of melanoma patients with sorafenib results in compensatory activation of non-MAP kinase pathways. Similarly, mTOR inhibition due to rapamycin and derivatives has been shown to result in compensatory Akt activation (44).
Tris-DBA has the advantage that it inhibits several pathways required for melanoma tumorigenesis, including MAP kinase activation, phosphoinositol-3 kinase/akt activation, stat-3 activation, and S6 kinase activation. These pathways are involved in virtually all forms of human cancer, as well as autoimmune disorders, such as lupus, eczema (atopic dermatitis), asthma, psoriasis and arthritis. While no drug is likely to be completely effective as monotherapy in melanoma, Tris-DBA is well tolerated systemically in mice, and has a novel profile of action compared with other clinically used chemotherapeutic agents. Its ability to inhibit phosphoinositol-3 kinase activation may enhance the activity of cytokines which require akt inactivation for optimal activity, and may enhance the activity of other chemotherapeutic agents. Tris-DBA may also enhance the activity of sorafenib by blocking compensatory signaling that may occur in vivo due to MAP kinase blockade. Our studies provide a rationale for the further investigation of Tris-DBA in the treatment of malignant melanoma.